When it comes to covering up a flat roof, your options are both limited and expansive. What that means in a nutshell is that your traditional roofing materials such as asphalt shingles, concrete tiles, and corrugated metal are out of the window. That said, flat roof systems such as PVC, TPO, EPDM rubber, and others, each have their distinct pros and cons.
So why can’t you put the traditional roofing materials like asphalt shingles on a flat roof? Well, technically you can, but they are almost guaranteed to leak!
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Systems designed for sloped roofs, such as asphalt shingles and concrete or clay tiles, are installed by overlapping one row on top of another. These systems work cohesively with the pitch of the roof to shed rainwater and snow as it falls from the sky.
Since flat roofs have little to no pitch, the water would work itself underneath the shingles or tiles, eventually rotting the substrate and causing leaks on your interior.
Covering a flat roof is a whole different animal (specialized skills and expertise are required) than shingling a pitched one. On a flat roof you want to avoid any types of seams, if possible.
The biggest threat is of course going to be water, which WILL find any access though any hole or inadequately sealed seams in a roofing membrane.
Your main goal when covering a flat roof is to create a barrier that will be impenetrable to water.
How do you make an item (besides a roof) impenetrable to water? You can either apply something physical such as a tarp or coat it with a material to create a barrier like you would via deck stain or lacquer. Roofs follow this same premise –- either physically cover them with something like a PVC or an EPDM rubber membrane or apply a coating such as tar or spray on silicon.
Flat Roof Costs:
It’s easy to assume that a flat roof would be far less expensive to apply materials to than a pitched one. For access reasons alone, it would seem it’s a lot easier to roof a flat surface than one that is steep and requires a harness and a rope line to move around.
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Many roofing contractors will tell you, though, that working on flat surfaces is actually harder on the back.
Take into account that flat roofs often require the application of a layer of insulation, adhesives or hot air welding, and some types such as modified bitumen are installed with a torch that heats up the seal.
Installation costs can be just as high for a flat roof as they would be for a steeper roof with say an 18/12 pitch.
Professional warrantied labor prices will vary across different markets in the US. You can get a fairly good idea of the estimated cost to cover a flat roof based on the national average: